This invention is concerned generally with bipolar battery structures and more particularly is directed to a bipolar battery which accommodates electrolyte volume changes during battery cycling, allows easy assembly of a large number of cells and includes corrosion resistant sulfide ceramic seals.
Current lithium alloy/iron sulfide bipolar batteries have positive and negative electrode materials which are confined relative to the collectors of positive and negative current. The current collectors are electrically insulated from one another by a separator element. Typically, the negative electrode material is a lithium alloy, such as, LiAl or LiSi, and the positive electrode material is an iron sulfide, such as, FeS or FeS.sub.2. The separator elements are formed of a fibrous boron nitride, a pressed powder magnesium oxide or an aluminum nitride. An electrolyte, such as a lithium chloride, lithium bromide and potassium bromide mixture, is present in the electrode materials and the separator element. The positive and negative current collectors are commonly formed of electrically conductive sheets that also confine the electrode materials.
A substantial amount of development work has been performed in the area of high-temperature, secondary electrochemical cells. Positive electrodes for these cells have included chalcogens, such as sulfur, oxygen, selenium or tellurium and further have included transition metal chalcogenides.
In high-temperature cells, current flow between electrodes often is accomplished by means of molten electrolytic salts. Particularly useful salts include compositions of the alkali metal halides and/or the alkaline earth metal halides which ordinarily incorporate a salt of the negative electrode reactant metal, e.g., lithium. In cells operating at moderate temperatures, aqueous and organic base electrolytes are permissible, and these also can include cations of the negative electrode metal.
Alkali metals such as, lithium, sodium, potassium, or alkaline earth metals, including calcium and magnesium and alloys of these materials, are contemplated as negative electrode reactants. Alloys of these materials, such as lithium-aluminum, lithium-silicon, lithium-magnesium, calcium-magnesium, calcium-aluminum, calcium-silicon and magnesium-aluminum, have been utilized to maintain the negative electrode in solid form and thereby improve retention of the active material at high cell operating temperatures.
Full size batteries are comprised of many cells grouped together in an end-to-end, or face-to-face, arrangement in a common battery housing and are electrically connected in series to produce higher effective voltage output. A thin cell version is capable of very high current density, and the battery is designed to operate at temperatures in the range of 375.degree.-500.degree. C. The electrode materials and also the electrolyte are quite corrosive at these operating temperatures so that the current collectors must be corrosion resistant, yet must function as electrically conductive material. Moreover, the battery is intended to have an operating life in excess of one thousand "deep discharge" cycles, where each such cycle involves discharging the fully charged battery to approximately a 5% charge level before recharging again. During this deep discharge cycling, the positive and negative electrode materials undergo volumetric changes at different rates. This difference in volume change can shift the electrode materials relative to one another within the battery cell or can even deform the separator elements.
Another major problem in existing bipolar battery designs, particularly those involving electrolytes normally fluid at cell operating temperatures (i.e., 375.degree.-500.degree. C.), has been electrolyte leakage past the wetted separator element present between the adjacent positive and negative electrodes. The electrolyte is consumed by electrolytic decomposition and could produce metallic deposits sufficient to cause battery failure by shorting out the adjacent collectors or shorting to the external battery housing.
Compression bonding of sandwiched plate-like cell components within the battery case is currently used in many bipolar batteries as the primary means to maintain the edge of the separator elements sealed. This is accomplished prior to introduction of electrolyte to the cells. U.S. Pat. No. 4,687,717 (incorporated by reference herein) describes various approaches to hermetic sealing of the bipolar battery by forming thermal compression seals for each cell. The required degree of compression for seal formation has limited the design flexibility for improving battery performance and for improving the economics of battery fabrication.
It is therefore an object of the invention to provide an improved design for a bipolar battery.
It is another object of the invention to provide a new bipolar battery with leak proof sulfide ceramic seals of the positive and negative electrode materials.
A further object of the invention is to provide an improved bipolar battery cell in which seals adjacent the electrolyte cavity or chamber are formed prior to the addition of electrolyte.
It is yet another object of the invention to provide an improved battery design having a repeating, cup-like design which allows the battery components to be assembled incrementally from the open end of a sealed structure.
A further object of the invention is to provide an improved bipolar battery cell in which the telescoping seal elements capture the periphery of the separator of the cell element to enhance cell durability.
It is still an additional object of the invention to provide a new battery containment design having the positive and negative current collecting containment elements of adjacent cells sharing a common bipolar current collector to provide a series electrical cell connection.
It is yet a further object of the invention to provide a novel battery current-collector element which connects the dispersed current-collector to the planar current-collector via a conductive ceramic transition element to significantly reduce interfacial resistance.
It is yet a further object of the invention to provide a novel battery current-collector element which is displaceable to accommodate changes in electrode volume during electrical charging and discharging cycles.
It is yet another object of the invention to provide an improved bipolar battery structure having refractory metal coated steel components for containing the electrodes.
It is still an additional object of the invention to provide an improved bipolar battery structure having a combination of external ceramic rings and sealants which encapsulate the battery to resist humidity and provide electrical insulation.
It is yet an additional object of the invention to provide a method for forming high strength bonds between metals (especially refractories) and sulfide ceramics or solder glasses by modifying the bond interface to form an intermetallic material such as an aluminide, silicide or phosphide.